The ferrite bead is needed on the shield to reduce EMI. It will absorb
circulating currents flowing through the shield returning through the
system and rack grounds. Since it is absorptive rather than inductive
it actually terminates this energy. It should be designed to absorb power
at a frequency associated with the edge rates of the signal. And it shoud
be a reasonable short at the data rate. An open at one end will eliminate
low frequency (<1 Mhz) power chopper ground currents. Capacitance can
ensure connection at higher frequencys.

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